1. Field of the Invention
The present invention relates to a reproducing apparatus of a signal recorded on a tape-like recording medium.
2. Related Background Art
Hitherto, a recording and reproducing apparatus for recording and reproducing a digital signal onto a magnetic tape by the helical scanning method has been well known. Further, there has also been known an apparatus with a construction which realizes both a normal recording mode of a normal recording time and a long time recording mode, which can execute the recording for a longer time by reducing the number of data tracks recorded in a unit time, although picture quality slightly deteriorates (for instance, JP-A-63-306504).
FIG. 1 shows a constructional block diagram of a conventional example of a digital signal recording and reproducing apparatus having both the normal mode and the double mode which can record for a long time, i.e. twice as long as the normal recording time. An analog video signal of the interlace scan in which one frame is constructed by two fields is input to an input terminal 10. An A/D converter 12 samples the input video signal by a sampling frequency, which is two or more times as high as the highest frequency of the video signal, and converts it into a digital signal of, for instance, eight bits. An output of the A/D converter 12 is once stored into a frame memory 14, which eliminates a blanking period is eliminated. On the other hand, in the double mode, a time base conversion is executed so as to match with a recording timing, which will be explained later herein. The signal from which the blanking period was eliminated by the frame memory 14 is supplied to a multiplexer 20 through a switch 18 in the normal mode. In the double mode, the signal from the frame memory 14 is compressed by a DPCM encoding circuit 16 and, thereafter, it is applied to the multiplexer 20 via the switch 18. The DPCM encoding circuit 16 non-linearly digitizes the difference between the present pixel and the preceding pixel by use of, for example, a correlation of images, thereby compressing the 8-bit digital signal into a the 4-bit signal in the amplitude direction and reducing an information amount into 1/2.
The multiplexer 20 divides the input signal into the signals of two systems. Error correction code (ECC) addition circuits 22A and 22B add error correction codes to outputs of the two systems of the multiplexer 20. An ID generation circuit 24 generates a signal in which a strong error correction code was added to identification (ID) data such as present recording mode, field number, scanning line number, and the like. An output of the ID generation circuit 24 and outputs of the ECC addition circuits 22A and 22B are multiplexed on the time base by switches 26A and 26B. That is, the switches 26A and 26B are switched so that one ID data is added to the data of a few scanning lines.
Digital modulation circuits 28A and 28B execute a low band suppressing modulation such as an 8-9 modulation or the like to signals from the switches 26A and 26B. Outputs of the digital modulation circuits 28A and 28B are supplied to magnetic heads HA+, HB+, HA- and HB- through amplifiers 30A and 30B, change-over switches 32A and 32B for recording/reproduction, and head selecting switches 34, 36, 38, and 40 and are recorded onto a magnetic tape 42.
FIG. 2 shows a timing chart of switching control signals S.sub.1, S.sub.2, S.sub.3, and S.sub.4 of the switches 34, 36, 38, and 40. FIG. 2A shows the case of the normal mode. FIG. 2B shows the case of the double mode. d.sub.1 and d.sub.2 indicate signals of common contact portions of the switches 32A and 32B. When the control signals S.sub.1, S.sub.2, S.sub.3, and S.sub.4 are at the high level, the corresponding switches 34, 36, 38, and 40 are turned on. A hatched portion in FIG. 2B shows that a signal to be recorded does not exist. The time base compression to form the hatched portion is executed by the frame memory 14.
FIG. 3A is a plan view of a rotary cylinder having the magnetic heads HA+, HB+, HA- and HB-. FIG. 3B is a development diagram in the circumferential direction of the rotary drum. As shown in FIG. 3A, the magnetic tape 42 is wrapped around the outer peripheral surface of a rotary cylinder 44 by an angle range of 180.degree. or more. The heads HA+, and HB+ attached to the rotary cylinder 44 have a plus azimuth and the heads HA- and HB- have a minus azimuth. The heads HA+ and HB+ rotate with a phase difference of 180.degree.. The heads HA- and HB- also similarly rotate with a phase difference of 180.degree.. On the other hand, the heads HA+ and HB- rotate with a phase difference of an angle .theta..
The operations of the heads in the normal mode and the double mode will now be described. The rotary cylinder 44 rotates two times for a period of time when the video signal of one frame is input to the input terminal 10. That is, in the case of the NTSC signal, a rotational speed of the cylinder 44 is set to 3,600 rpm. In the normal mode, each of the magnetic heads HA+, HB+, HA- and HB- traces the magnetic tape 42 two times for a period of time when the cylinder 44 rotates twice, thereby recording the signal onto the magnetic tape 42. During such a period of time, the magnetic tape 42 is conveyed at a predetermined speed in a manner such that eight tracks are formed in one frame at a predetermined track pitch T.sub.p by a capstan 48 which is controlled by a capstan control circuit 46.
In the double mode, the signal is recorded onto the magnetic tape 42 by the tracing operations of two times of the magnetic heads HA+ and HA- for a period of time when the cylinder 44 rotates twice. The magnetic heads HB+ and HB- are not used. If the tracing direction of the head is set at small enough an inclination angle for the longitudinal direction of the magnetic tape 42, the conveying speed of the magnetic tape 42 by the capstan 48 is 1/2 of that in the normal mode and the number of tracks which are formed in one frame is set to four.
FIG. 4A shows a track pattern in the normal mode. FIG. 4B shows a track pattern in the double mode. In FIGS. 4A and 4B, TA+, TB+, TA- and TB- respectively indicate tracks which were recorded by the magnetic heads HA+, HB+, HA- and HB-.
A system control circuit 50 in FIG. 1 controls the foregoing recording operation and also controls the reproducing operation, which will be explained later herein.
In the reproducing mode, the switches 32A and 32B are connected to the P contact side and the switches 34, 36, 38, and 40 are connected or disconnected at timings similar to those in the recording mode. Thus, reproduction signals of two systems can be derived from the P contacts of the switches 32A and 32B. The reproduction signals are amplified by reproducing amplifiers 52A and 52B and demodulated by digital demodulation circuits 54A and 54B. ECC decoding circuits 56A and 56B error-correct the demodulation outputs of the digital demodulating circuits 54A and 54B. A composition circuit 58 synthesizes error correction outputs of the ECC decoding circuits 56A and 56B, thereby obtaining the original signal of one system. On the other hand, an ID decoding circuit 60 extracts and decodes the ID data portions from the outputs of the modulation circuits 54A and 54B.
In the case of the signal recorded in the normal mode, an output of the composition circuit 58 is supplied to a frame memory 66 through a switch 64. In the case of the signal recorded in the double mode, the signal is decoded by a DPCM decoding circuit 62 and, thereafter, it is supplied to the frame memory 66 via the switch 64. The DPCM decoding circuit 62 expands the signal which was compressed into four bits in the amplitude direction into the original 8-bit signal.
The frame memory 66 stores the input reproduction data into proper pixel positions in accordance with the ID data from the ID decoding circuit 60. Since only the effective pixels are input as the reproduction data into the frame memory 66, the blanking period is added here. On the other hand, in the double mode, there is executed a time base conversion for reconstructing the data which is intermittently reproduced into a predetermined image signal. An output of the frame memory 66 is converted into an analog signal by a D/A converter 68 and is output to the outside through an output terminal 70.
In the above conventional example, if the operator wants to execute a special reproduction such as a searching operation at a speed different from the speed upon recording, the simplest method is to change the speed of the capstan 48 by generating a common from the system control circuit 50 to the capstan control circuit 46. For instance, in FIG. 5A, in the case where the magnetic tape which had been recorded in the normal mode was searched at a speed which is 4.6 times as high as the speed in the normal mode, a portion from which data can be reproduced in a state in which the azimuth angle of the recording track coincides with the azimuth angle of the reproducing head is shown as a hatched region. #1 to #7 show relative field numbers to which the reproduced data belongs. Since the ID data including the scanning line numbers and the like have been recorded together with the image data, it is possible to discriminate between the position of the scanning line on the display screen where the image data is located from and the ID data which was reproduced and decoded, so that the reproduction image data can be written at the proper location in the frame memory 66.
On the other hand, FIG. 5B shows the case where the magnetic tape recorded in the double mode was searched at a speed which is 4.6 times as high as the speed in the double mode. A region from which data can be reproduced is shown by a hatched region. #1, #2, #3, #6, and #7 indicate relative field numbers to which the reproduced data belongs. As will be understood from FIG. 5B, a time interval of reproducible data is fairly larger than that in the normal mode. As mentioned above, in what is called a long-time mode for recording by reducing the information amount, the images which are recorded are the partial images lacking absolute information. Therefore, there is a problem in that the data which can be reproduced in a special reproducing mode is also the further partial data, and the reproduced image becomes unrefined.